The widespread high-performance image sensors including charge coupled device (CCD) sensors have caused an increased need for techniques to remove noise from image signals. An image sensor having higher pixel count causes electric currents flowing between adjacent pixels to affect one another, contributing to generation of noise. On the other hand, images captured by high pixel-count image sensors are steadily growing in data size. Raw image data is not suitable to be processed, and thus image signals are compressed. However, a compression process can generate some noise such as what is called mosquito noise or block noise.
PTL 1 discloses an image processing device which carries out coring processing to reduce noise while suppressing blurred edges and phase shifting caused by the coring. PTL 2 discloses an image processing device which enables to reduce noise in a whole image including edges and their surrounding areas while preventing edges from blurring in an image. PTL 3 discloses an image processing device which enables to remove noise while reducing blurred edges caused by coring processing. PTL 4 discloses an image processing device which achieves an inexpensive noise removing circuit with the circuit scale largely reduced while providing optimum noise reduction with respect to luminance and color difference respectively.
In addition, PTL 5 discloses total variation (TV) denoising. TV denoising is a technique to remove noise by using color information in surrounding pixels while preserving edge components.